1. Field of the Invention
The present invention generally relates to semiconductor devices, and particularly relates to a semiconductor device which allows a device type thereof to be switched.
2. Description of the Related Art
When semiconductor devices are manufactured, they are generally designed to be adaptable to various device types (different operational configurations), and one of the device types is selected and fixed before shipment of the semiconductor devices. For example, an input-output interface is designed such that the input-output interface can be switched between a 4-bit configuration, an 8-bit configuration, a 16-bit configuration, and a 32-bit configuration, thereby allowing a choice to be made at the time of shipment as to the input-output-interface configuration of the semiconductor device. Further, a memory device may be provided with a choice as to how many refresh commands should be input in order to refresh the whole memory cell array, and a desired configuration is selected at the time of shipment of the device.
Such a semiconductor device provided with a selectable device type conventionally uses pads for selecting a device type. This selection is made by bonding a selected pad to a power voltage VCC or a ground voltage VSS.
FIG. 1 is a circuit diagram of a device-type switching mechanism of the related art.
A device-type switching decoder of FIG. 1 includes NAND circuits 201 through 204, inverters 205 through 210, and pads P1 and P2. Depending on what combination of a HIGH (VCC) voltage and a LOW (VSS) voltage is applied to the pads P1 and P2, one of the inverters 205 through 208 outputs a HIGH signal. The outputs of the inverters 205 through 208 correspond to a 4-bit configuration, an 8-bit configuration, a 16-bit configuration, and a 32-bit configuration of an input-output interface, respectively. Before the shipment of the semiconductor device, the semiconductor device needs to be sealed inside a package. During the sealing process, the pads P1 and P2 are bonded to the power voltage VCC or the ground voltage VSS. After this bonding, a selected one of the inverters 205 through 208 outputs a HIGH signal when the device is operated. The outputs of the inverters 205 through 208 are supplied to an internal circuit (including an input-output interface circuit) of the semiconductor device, so that the semiconductor device operates based on the selected input-output-interface configuration.
A pad occupies a large area. When pads are used for the switching of a device-type, chip size is bound to increase because of the incorporation of the ability to switch a device type. Within a limited chip size, it is difficult to secure a space for these pads, especially when the number of I/O signals is large.
Moreover, there is a need to examine output signals from a semiconductor device while providing various input signals to the semiconductor device in order to make a final check on operations after the bonding is put in place. When a device type is fixed to a 4-bit configuration by bonding pads, for example, the number of input-output-interface bits is one fourth of that of a 16-bit configuration. When the 4-bit configuration is selected, therefore, the check requires a test time four times as long as that of the 16-bit configuration.
Accordingly, there is a need for a semiconductor device which does not use pads for switching a device type.
There is another need for a semiconductor device which can switch a device type even after a device type is once fixed.
There is yet another need for a method of testing a semiconductor device which requires a relatively short time period when the semiconductor device described above is used.